It is common practice to use, in athletic exercise apparatus, cams by the aid of which a load of varying magnitude, depending on the degree of flexion of a joint, can be imposed on the joints and muscles which have to be exercised. Such structures are however embarrassed by the problem that one and the same cam cannot be used in exercising both the extensor and flexor muscles of an extremity because the functions of the joint at extension and at flexion are not directly equivalent in the sense that the relative loads required at a given flexion of the joint would be equal. Therefore already the initial situation, that is the distance of the starting point of the cam's guiding surface from the centre of rotation of the cam is not the same when the extremity is extended and when it is flexed. One is therefore compelled in present art to employ separate athletic exercise units when training the extensor musculature and the flexor musculature.